Recent advances in the field of optical communication with new, more complex, data modulation formats as a key technology has created a need for optical waveform characterization tools that are capable of extracting more information from the waveform than simply its power as a function of time. Encoding data onto an optical carrier by modulation of both the optical field phase and amplitude has become increasingly attractive and appears to be a technological approach that will contribute to increase the capacity of future fiber optic communication links.
However, the need to measure the complete electric field of an optical signal—which is necessary to visualize both the phase and amplitude information of the signal—requires coherent detection techniques that utilize a reference phase at the measurement point. In most cases, coherent detection utilizes a continuous-wave (CW) local oscillator (LO) reference signal generated by a separate, independent laser source. The ability to “mix” the optical input signal (carrying optically-encoded data) with such a reference signal opens up the possibility of measuring the data-related, time-varying phase change of the optical input signal relative to the reference LO signal.
Coherent detection is not a novel technology; in fact, it was extensively studied during the 1980's and proposed as a solution for high-sensitivity optical-signal detection. However, implementation was difficult and with the advent of erbium-doped fiber amplifiers (EDFAs), the commercial deployment of coherent detection systems was delayed. Nevertheless, research has continued in the field and recently attracted new interest, driven by the need for more spectrally-efficient modulation formats, as well as the availability of high-speed electronic processing for post-reception compensation of transmission impairments.
The transition towards novel, advanced modulation formats for optical communication has evolved into incorporating the modulation of both amplitude and phase, creating a need for new measurement technologies that are capable of measuring the time-varying electric field of an optical input signal. In particular, the coherent detection of high-speed optical input signals will require measurement systems with a relatively large bandwidth for accurate signal reconstruction. Digital sampling is a technique that can provide sufficient bandwidth for this purpose.
Indeed, digital sampling is a well-known technique used to visualize a time-varying waveform by capturing quasi-instantaneous snapshots of the waveform via, for example, a sampling gate. The gate is “opened” and “closed” by narrow pulses (strobes) in a pulse train that exhibit a well-defined repetitive behavior such that ultimately all parts of the waveform are sampled. The sampling implementation can either be “real-time” or “equivalent-time”, where real-time sampling refers to the case where the sampling rate is higher than twice the highest frequency component of the waveform (Nyquist sampling) and equivalent-time sampling refers to the use of an arbitrarily low sampling rate with a “repetitive” waveform to provide accurate signal reconstruction. The need for a “repetitive” waveform is a fundamental limitation of the equivalent-time sampling approach.
There are several digital sampling-based coherent detection systems in the prior art that facilitate characterization of the electric field of a data-carrying optical input signal by coherent mixing with a reference signal and subsequent signal processing (for signal reconstruction and visualization). Selected prior art solutions are outlined hereinbelow, including an identification of particular limitations that will be addressed by the teachings of the present invention.
FIG. 1 shows an exemplary prior art optical detection arrangement 1 for measuring the electric field of a data-varying optical input signal S (actually, a signal S(t) comprising a time-varying optical carrier oscillating at the optical carrier frequency; for the sake of simplicity referred to hereinafter as “S”) by coherent mixing of its electric field with the electric field of a known CW local oscillator reference signal LO in an optical hybrid element 2. Optical hybrid 2 functions to mix these two signals, S and LO, in the complex-field space to create a set of four mixed-field optical signals: S+LO, S−LO, S+jLO and S−jLO, as shown in FIG. 1.
A square-law detection function is then applied to these signals to convert them into photodetector currents (electrical signals) to be sampled and studied. Preferably, a “balanced detector” arrangement is used that allows for intermediate frequency (IF) terms to cancel. As shown in FIG. 1, a first pair of mixed-field optical signals S+LO and S−LO are applied as separate inputs to a first balanced detector 3-A and, similarly, a second pair of mixed-field optical signals S+jLO and S−jLO are applied as separate inputs to a second balanced detector 3-B. The pair of output detector currents from balanced detectors 3-A and 3-B, respectively, can be expressed as:I1(t)=4|S(t)∥LO|cos(ωIFt+φs(t)+φLO,1), andI2(t)=4|S(t)∥LO|cos(ωIFt+φs(t)+φLO,2),where ωIF=ωs−ωLO and is defined as the frequency difference between the electric fields of signals S and LO, φs(t) is the time-varying phase of the optical input signal (associated with the particular data pattern of the signal) and the quantity (φLO,1−φLO,2) is defined as the “relative phase shift” between each of the output signals from optical hybrid 2. In the preferred embodiment of the prior art, optical hybrid 2 is constructed to maintain a 90° (i.e., π/2 radian) phase shift between adjacent outputs.
Referring again to FIG. 1, the pair of photocurrents I1(t) and I2(t) are thereafter amplified through a pair of amplifying devices 4-A and 4-B before being digitally sampled in a pair of analog-to-digital (A/D) converters 5-A and 5-B to generate streams of output samples, shown as O1 and O2 in FIG. 1. Finally, digital streams O1 and O2 are applied as inputs to a signal processor 6, which functions to create a visualization of the electric field of optical input data signal S, based on the digital sample streams.
In most arrangements, separate laser sources are used to generate data-carrying optical input signal S and reference signal LO, so that ωIF≠0. Thus, the IF needs to be calculated by processor 6 in order to extract φs(t), the data-related phase modulation of optical input signal S. Once the IF portion is removed, it is straightforward to extract both the amplitude and phase information of optical input signal S and thereby visualize the measured signal in a convenient manner (for example, a constellation diagram).
Prior art coherent detection arrangement 1 as shown in FIG. 1 employs an electronic sampling technology that has at least one significant drawback—the bandwidth limitation of the electronic A/D converter and sampling circuits (i.e., for “electronic sampling”). Indeed, the highest available analog bandwidth in today's A/D converters is on the order of 20 GHz (at best); therefore, the maximum measurable signal baud is around 10 GBaud.
FIG. 2 shows another exemplary prior art arrangement for a coherent detection, in this case comprising a linear optical sampling system that is capable of measuring the complete electric field of the optical input signal. The configuration is similar to that of FIG. 1, with optical hybrid 2, balanced detectors 3, amplifiers 4 and A/D converters 5 all functioning as discussed above.
In this case, optical input signal S is mixed in optical hybrid 2 with a reference signal LO originating from a “pulsed” sampling laser source 7. The main difference between the prior art linear sampling system in FIG. 2 and the electronic sampling system in FIG. 1 is the utilization of a “pulsed” LO signal source in the arrangement of FIG. 2. In contrast, a continuous wave (CW) source is employed in the prior-art arrangement of FIG. 1. Pulsed sampling laser source 7 thus serves as a source of both reference signal LO for coherent detection and a strobe signal enabling a fast optical gating functionality that is independent of the limited bandwidth of A/D converter 5. By reducing the optical sampling rate to well below the analog bandwidth of A/D converter 5, the “equivalent” measurement bandwidth of the overall system will be dictated only by the temporal resolution of the optical sampling gate (roughly the pulse width of the pulsed LO reference source 7, advantageously on the order of a few picoseconds or less).
As before, the four output mixed electric-field signals from optical hybrid 2 are applied as inputs to a pair of balanced detectors 3-A and 3-B. The output detector currents I1(t) and I2(t) are then amplified by amplifiers 4-A and 4-B and applied as separate inputs to A/D converter 5. In this prior art linear sampling system, A/D converter 5 needs to operate at the same sampling rate as pulsed sampling laser source 7. To accomplish this, a photodetector 8-D and a pulser circuit 8-P are coupled in series between pulsed sampling laser source 7 and A/D converter 5 and used to create a clock signal that synchronizes the sampling rate of pulsed sampling laser source 7 with the sampling rate of A/D converter 5. With acquired batches of samples of the photodetector currents from A/D converter 5, the required signal processing needed in order to reconstruct the original waveform is similar to that described for the electrical sampling case in FIG. 1 and is not explicitly illustrated in FIG. 2.
There remains, however, a few drawbacks with this hardware implementation, particularly related to strict wavelength requirements on the pulsed sampling laser source. That is, the linear optical sampling technology requires the sampling-pulse spectra to overlap the optical signal spectra in order to provide distortion-free gating and coherent mixing using the same laser source. This requirement complicates the possibility of providing an optically broadband measurement system, since if the wavelength of the optical input signal is changed, the pulsed sampling laser source must also adapt its wavelength.
Another parameter that is even more challenging is the fact that the pulse-to-pulse phase stability of pulsed sampling laser source 7 must be very stringent; that is, each pulse conserves a phase relation with the preceding pulse as if one were “pulse carving” narrow, temporal slices (e.g., linewidth <1 MHz) of the output from a CW laser source at the sampling rate. While such low-phase-noise sources are known in the art (e.g., passively-modelocked fiber ring lasers), their repetition rate is generally less than about 20 MHz. Indeed, there are no tunable commercial, suitably low-phase-noise short-pulse sources capable of offering the typically >500 MHz sampling rate that is most desirable for robust IF recovery algorithms.
Thus, a need remains in the art for an arrangement capable of characterizing (visualizing) the complete electric field of high symbol rate (“baud”) optical signals without being hampered by limited electrical measurement bandwidth or by the need for unnecessarily complicated optical sampling pulse sources.